In order to achieve a rapid ultimate strength testing of test samples exposed to tensile stress it is necessary to make sure that the loading velocity rises steeply at the beginning of the loading stroke to rapidly reach a predetermined loading velocity value. Conventional equipment for a rapid ultimate strength testing is provided with a lead stroke mechanism which establishes an acceleration or lead distance along which the load application device is accelerated to reach the predetermined velocity value. After passing through the lead distance, the stationary sample or body to be tested is coupled to the moving load application device and a loading force is thus transmitted to the sample or body to be tested.
In connection with conventional lead stroke devices problems have arisen at the instance of coupling the test sample to the load applicator after passing through the lead distance. These problems are due to the fact that the mass moved by the load applicator impacts on a stationary mass represented by the machine frame components connected to the test sample to be subjected to the ultimate tensile strength test. This impact is an elastic impact, whereby the load application device may be decoupled from the test sample, especially since a load application member must be accelerated for applying the test load. As a result, the respective masses are accelerated to different velocities. The decoupled component determines the required energy for breaking or rupturing the sample. This energy in turn depends on the decoupling speed and on the mass of the components involved.
European Patent Publication 0,314,829 (Beran et al.), published on May 10, 1989, relates to a machine for the rapid ultimate strength testing, wherein the above mentioned undesirable decoupling is substantially prevented in that the test sample to be exposed to an ultimate tensile load is provided with clamping jaws having outer wedge surfaces for cooperation with respective wedge surfaces arranged on the load application device or on a machine frame. Preparing the test samples with the wedge-shaped clamping jaws is rather expensive because the wedge surfaces on the test sample and the wedge surfaces on the counter-bearings require high precision machining work.
U.S. Pat. 4,426,875 (Crosby), granted on Jan. 24, 1984, discloses a strain measurement apparatus for measuring the elongation of a rope or cable under impact stress by attaching a light emitting diode to a free-falling weight which loads the rope upon impact. The test sample is connected with its upper end to the machine frame and the lower end of the test sample is provided with a further clamping device. During the testing the load application velocity required for the testing is achieved by a loading plate which is releasably arranged between the two ends of the test sample, and which is loaded with weights. The so loaded plate impacts onto the lower clamping device of the test sample after the plate and its weights pass through the lead distance.
U.S. Pat. No. 3,407,651 (Sophy), issued on Oct. 29, 1968, relates to a high speed tensile testing machine in which a test piece is subjected to a high tensile force exerted by a piston. The motion of the piston which is exposed to pressure, is initially prevented by a frangible member which is then rapidly fractured by the detonation of an explosive charge within the frangible member, whereby the pressurized piston is rapidly accelerated.
U.S. Pat. No. 3,102,421 (Cosner et al.) issued on Sep. 3, 1963, discloses a high speed tensile testing apparatus in which one end of the test sample is held by an anchor jaw and the other end is held by a clamping jaw on a freely movable specimen cross-head. A load sensor is installed between the specimen anchor jaw and a specimen bed rigidly attached to the main frame of the machine to provide a continuous stress sensing during a testing operation. When the hydraulic fluid flow rate corresponding to a desired test velocity is attained, a solenoid latch energizes and releases the driving cross-head, thereby permitting movement of an actuating assembly for applying the required tensile stress to the specimen or sample.
An article entitled: "A New Generation of High Speed Tensile Testing Machines" by Kussmaul et al., published in "Measurement", Volume 9, No. 2, April-June, 1991, discloses a high speed tensile testing machine for large scale specimens. Testing loads up to 12 Mega-Newtons (MN) may be applied to determine the influence of high loading rates on the stress and strain behavior of unwelded and welded components of ferritic and austenitic materials. The maximum tensile force is generated by a propellant charge with a piston velocity of 25 m/sec. after a lead stroke of 2 mm or a maximum velocity of 60 m/sec. after a lead stroke of 400 mm.